1. Field of the Invention
The present invention relates generally to prosthetic devices, and more particularly, to a mechanical fixation system for a prosthetic device.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive or sensorineural. In many people who are profoundly deaf, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. Various prosthetic hearing implants have been developed to provide individuals who suffer from sensorineural hearing loss with the ability to perceive sound. One such prosthetic hearing implant is referred to as a cochlear implant. Cochlear implants use an electrode array implanted in the cochlea of a recipient to bypass the mechanisms of the ear. More specifically, an electrical stimulus is provided via the electrode array directly to the cochlea nerve, thereby causing a hearing sensation.
Conductive hearing loss occurs when the normal mechanical pathways to provide sound to hair cells in the cochlea are impeded, for example, by damage to the ossicular chain to ear canal. However, individuals who suffer from conductive hearing loss may still have some form of residual hearing because the hair cells in the cochlea are generally undamaged.
Individuals who suffer from conductive hearing loss are typically not candidates for a cochlear implant due to the irreversible nature of the cochlear implant. Specifically, insertion of the electrode array into a recipient's cochlea results in the destruction of the majority of hair cells within the cochlea. The destruction of the cochlea hair cells results in the loss of all residual hearing by the recipient.
Rather, individuals suffering from conductive hearing loss typically receive an acoustic hearing aid, referred to as a hearing aid herein. Hearing aids rely on principles of air conduction to transmit acoustic signals through the outer and middle ears to the cochlea. In particular, a hearing aid typically uses an arrangement positioned in the recipient's ear canal to amplify a sound received by the outer ear of the recipient. This amplified sound reaches the cochlea and causes motion of the cochlea fluid and stimulation of the cochlea hair cells.
Unfortunately, not all individuals who suffer from conductive hearing loss are able to derive suitable benefit from hearing aids. For example, some individuals are prone to chronic inflammation or infection of the ear canal and cannot wear hearing aids. Other individuals have malformed or absent outer ear and/or ear canals as a result of a birth defect, or as a result of common medical conditions such as Treacher Collins syndrome or Microtia. Furthermore, hearing aids are typically unsuitable for individuals who suffer from single-sided deafness (total hearing loss only in one ear) or individuals who suffer from mixed hearing losses (i.e. Combinations of sensorineural and conductive hearing loss).
These individuals who cannot benefit from hearing aids may benefit from hearing prostheses that use the principles of bone conduction device to provide acoustic signals to a recipient. Such hearing prostheses direct vibrations into the bone, so that the vibrations are conducted into the cochlea and result in stimulation of the hairs in the cochlea. This type of prosthesis is typically referred to as a bone conduction device.
Bone conduction devices function by converting a received sound signal into a mechanical vibration representative of the received sound. This vibration is then transferred to the bone structure of the skull, causing vibration of the recipient's skull. This skull vibration results in motion of the fluid of the cochlea, thereby stimulating the cochlea hair cells and causing a hearing sensation to be perceived by the recipient. Vibration from a bone conduction device is generally conducted to a recipient's cochlea via a screw implanted in the recipient's skull.
The skull bone, at the point of implant of the bone screw, is susceptible to damage from lateral forces on the bone screw, particularly during the healing period following the implant procedure. This healing period varies from person to person, depending upon many factors associated with the patient's overall health and genetics, but generally takes six weeks or more. During the healing period, the bone is so susceptible to damage that the general practice is that the bone conduction device is not coupled to bone screw until the healing period has ended. Further, while the bone is less susceptible to damage following the healing period, damage may still be possible if a large lateral force is applied thereto.